Bone anchoring element with thread that can be unscrewed

ABSTRACT

A bone anchoring element ( 1 ) for anchoring an external device in a bone is described. A head ( 2 ) capable of connection to the external device is provided with a shaft ( 3 ) that can be anchored in a bone. A predetermined section of the shaft ( 3 ) has a bone thread ( 5 ). Moreover, the shaft ( 3 ) also has at least one bone thread-free surface ( 6,7; 11,12; 16,17; 21,22,23 ) that extends from a first end to a second end of the predetermined section essentially along the direction of the shaft axis. The bone anchoring element ( 1 ), thus provided, is pressed or soft-hammered into a hole ( 8, 28 ) previously generated in the bone and fixed in the bone by rotation by a predetermined angle. If the bone anchoring element has to be removed this can be achieved by unscrewing it in the standard fashion. The disclosure also describes methods for using the bone anchoring element.

FIELD OF THE INVENTION

The present invention relates to a bone anchoring element for theanchoring of a mechanical device in a bone.

BACKGROUND OF THE INVENTION

A bone anchoring element is described in DE 42 34 118 in the form of apedicle screw. It comprises a shaft and a head that can be connected toa rod for interconnection of a plurality of pedicle screws. The shaft isprovided with a thread that allows the screw to be screwed into a bone.

DE 43 07 576 describes a bone anchoring element with a polyaxialconnection between the head of a bone screw and a rod.

Such bone anchoring elements are inserted into the bone by screwing-inwhich is a relatively time-consuming and strength-requiring process.Moreover, a classical screwing-in process may be associated with highpressure forces acting on the bone, which is undesirable in the case ofolder or pre-damaged bones.

For this reason, the classical process of screwing-in is lesswell-suited for certain clinical requirements, especially in pediatricsurgery, surgery at the cervical spine, and neurosurgery.

Therefore, it is desirable to provide a bone anchoring element, inparticular for use in pediatric surgery, surgery at the cervical spine,and neurosurgery, which can be inserted into the bone more rapidly, moreeasily, and with less force so as to avoid exerting damaging forces onthe bone, while providing for secure attachment.

SUMMARY OF THE INVENTION

The present invention provides a bone anchoring element for anchoring anexternal device in a bone. The bone anchoring element comprises a head(2) that can be connected to the external device and a shaft (3) thatcan be anchored in a bone. The shaft comprises a bone thread (5) in apredetermined section and at least one bone thread-free surface (6,7;11,12; 16,17; 21,22,23) that extends from a first end to a second end ofthe predetermined section essentially along the direction of the shaftaxis.

By providing an area that bears no bone thread it is possible to pressor insert the bone anchoring element into an attachment hole that waspreviously generated in the bone and then anchor the bone anchoringelement by rotating it by a predetermined angle without involving aprocess of screwing-in. If the bone anchoring element has to be removedthis can be achieved by unscrewing it in the standard fashion.

Preferred embodiments of a bone anchoring element in accord with theinvention have one or more of the following features:

-   -   the distance between the bone thread-free surface (6,7; 11,12;        16,17; 21,22,23) and the shaft axis is smaller than the distance        between the crest of the bone thread (5) and the shaft axis;    -   a multitude of bone thread-free surfaces (6,7; 11,12; 16,17;        21,22,23) is provided, preferably two or three;    -   the multitude of bone thread-free surfaces (6,7; 11,12; 16,17;        21,22,23) are arranged at equal distances from each other in a        circumferential direction;    -   at least one of the bone thread-free surfaces is provided as a        planar surface (6,7; 21,22,23);    -   at least one of the bone thread-free surfaces (11,12) is        provided to be concave in shape;    -   the bone thread-free surfaces (6,7; 21,22,23) extend parallel to        the shaft axis;    -   the bone thread-free surfaces (16, 17) extend from the first end        to the second end of the predetermined section in the form of a        helical section;    -   the predetermined section extends to the end of the shaft (3)        that is opposite to the head (2);    -   a tip (4) is provided at the end of the shaft (3) that is        opposite to the head (2);    -   at least one of the bone thread-free surfaces (6,7; 11,12;        16,17; 21,22,23) is roughened or coated in order to improve the        in-growth of bone;    -   the shaft (3) is made from a shape memory alloy;    -   the head (2) is provided as receiving member (31) for connecting        to a rod (30); and    -   the head (2) is connected to the shaft (3) either firmly or        articulated.

The invention also provides a method for using a bone anchoring elementfor anchoring an external device in a bone, the method comprisingproviding a bone anchoring element as described herein, having a firstcross-sectional shape forming a hole in the bone having a secondcross-section with dimensions equal to or slightly less than thedimensions of the first cross-sectional shape, inserting the boneanchoring element into the hole, and rotating the bone anchoring elementaround its longitudinal axis to a predetermined angle to engage the bonethread in the bone. A plurality of bone anchoring elements can beconnected to a rod for stabilization of vertebrae or bones. Optionallythe method includes a step of removing the bone anchoring element. Thestep of removing comprises a step of unscrewing the bone anchoringelement in the standard fashion.

Additional features and characteristics of the present invention areevident from the description of embodiments on the basis of the attacheddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a perspective view of a bone anchoring element according toa first embodiment of the present invention.

FIG. 2 shows a plan view looking at the tip of the bone anchoringelement of FIG. 1.

FIG. 3 shows a front elevational view of the bone anchoring element ofFIG. 1 (i.e., generally in the direction of the arrow A in FIG. 2).

FIG. 4 shows a side elevational view of the bone anchoring element ofFIG. 1 (i.e., generally in the direction of the arrow B in FIG. 2).

FIG. 5 shows a schematic representation of the function of the boneanchoring element according to the first embodiment.

FIG. 6 shows a plan view looking at the tip of a bone anchoring elementaccording to a second embodiment of the present invention.

FIG. 7 shows a perspective view of a bone anchoring element according toa third embodiment of the present invention.

FIG. 8 shows a perspective view of a bone anchoring element according toa fourth embodiment of the present invention.

FIG. 9 shows a plan view looking at the tip of the bone anchoringelement of FIG. 8.

FIG. 10 shows a schematic representation of the function of the boneanchoring element according to the fourth embodiment;

FIG. 11 shows a schematic representation of a monoaxial connectionbetween a bone anchoring element and a rod;

FIG. 12 shows a schematic representation of a polyaxial connectionbetween a bone anchoring element and a rod;

FIG. 13 shows a second embodiment of the invention;

FIG. 14 shows a third embodiment of the invention; and

FIG. 15 shows a further example of application of the bone anchoringelement according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

In accord with the present invention, bone anchoring elements areprovided that can be inserted into a bone without screwing the threadedlength of the shaft into the bone to the desired depth. A firstembodiment of a bone anchoring element in accord with the presentinvention is described with reference to FIGS. 1 through 5.

As is best seen in FIG. 1, the bone anchoring element 1 comprises a head2 and a shaft 3. Head 2 is provided to be spherical segment-shaped andcomprises means for engagement with a rotating tool for turning the boneanchoring element. Such means can comprise a slot, cross slot, hexagonalrecess, or outer hexagonal geometry, or similar device (not shown).

The shaft 3 comprises a tip 4 at its end opposite to the head. A bonethread 5 extends over the entire length of the shaft.

Two opposite sides of the shaft 3 are provided with two planar surfaces6, 7, which are parallel with respect to each other and to the shaftaxis (longitudinal axis of shaft 3) and extend over substantially theentire length of the shaft. The distance between surfaces 6, 7 issmaller than the core diameter of bone thread 5, such that surfaces 6, 7interrupt bone thread 5.

As is best seen in FIG. 2, the cross-section of the shaft 3 thus becomesapproximately rectangular, but has curved narrow sides that are formedby the thread portion. A first dimension of the cross-section isdetermined by the distance between surfaces 6, 7 and a second dimensionorthogonal to the first direction is determined by the outer diameter ofbone thread 5.

The bone anchoring element 1 typically is made of steel, titanium or anyother body-compatible and sufficiently stable material.

In order to insert anchoring element 1, first a corresponding oblonghole 8 is generated in the bone, the hole having a cross-sectional shapeas is best illustrated by the cross-hatched area in FIG. 5. This can beaccomplished, for instance, by drilling twin holes followed by removalof the separating wall with a rasp. Alternatively, the oblong hole canbe generated with a transverse cutter or profile-forming cutter. Theprofile of oblong hole 8 corresponds approximately to the cross-sectionof shaft 3. However, alternatively, it also can be slightly smaller.

Subsequently, shaft 3 of bone anchoring element 1 is inserted intooblong hole 8 by pressing or soft-hammering such that the longitudinalaxis of a sectional plane of shaft 3 perpendicular to the shaft axis isapproximately identical to the longitudinal axis t of oblong hole 8. Inthis arrangement, the bone anchoring element 1 is seated in oblong hole8 only so firmly that it can still be removed by pulling.

Subsequently, using a rotating tool engaged onto or into the head, boneanchoring element 1 is rotated by a pre-determined angle, preferably 90°for this embodiment, as depicted by the dashed line in FIG. 5. As aresult, the teeth of bone thread 5 now engage the bone and boneanchoring element 1 is firmly seated and fixed in the bone.

By rotating it further to an angle of 180° or by rotating back to theinitial position, if needed, bone anchoring element 1 can be loosenedand/or removed from the bone.

In the course of time, the bone grows into the empty spaces and, thusprovides additional stability to bone anchoring element 1.

In certain cases, it may be necessary to remove bone anchoring element 1from the bone after an extended period of time, when bone anchoringelement 1 is firmly surrounded by bone, e.g. if the anchored device isto be removed. At this time, simply rotating the bone anchoring element90° to its original insertion position and pulling out the element willno longer be possible. In this case, bone thread 5 serves for thepurpose of allowing bone anchoring element 1 to be removed from the boneby unscrewing with a rotating tool like a conventional bone screw.

FIG. 6 shows a second embodiment of a bone anchoring element in accordwith the present invention. In place of planar surfaces 6, 7, asillustrated for the first embodiment, shaft 3 of bone anchoring element1 comprises two opposite surfaces 11, 12, which are curved in thedirection of the shaft axis such that the cross-section of shaft 3 hasopposing concave longitudinal sides. In all other regards, the designand the use of the bone anchoring element 1 are identical to the firstembodiment.

Due to the concave surfaces 11, 12 the in-growth of bony material isimproved which can provide for more stability of the attachment of boneanchoring element 1 in the bone.

FIG. 7 shows a third embodiment according to the present invention.Here, in place of surfaces 6, 7 of the first embodiment or surfaces 11,12 of the second embodiment, shaft 3 of bone anchoring element 1comprises two opposite surfaces 16, 17 that are twisted around the shaftaxis by a predetermined angle a, such that they form a helix section. Inthis embodiment, at any cross section of the shaft perpendicular to theshaft axis the surfaces 16, 17 will show a straight line. In all otherregards, the design and the use of the bone anchoring element, 1, areidentical to the first embodiment.

Although the predetermined angle a is shown in FIG. 7 as being approx.90° for illustration purposes, the angle is preferably in the range of90°±45°. The slight twisting of surfaces 16, 17 also improves theanchoring of bone anchoring element 1 in the bone.

Similar to surfaces 11, 12 of the second embodiment, alternatively, thesurfaces 16, 17 of the third embodiment can also be concave.

A bone anchoring element according to a fourth embodiment according tothe present invention is described with reference to FIGS. 8 through 10.In place of the two parallel surfaces 6, 7, shaft 3 of bone anchoringelement 1 comprises three planar surfaces 21, 22, 23 which are off-setfrom each other by about 120° each, such that the cross-section of shaft3 becomes approximately triangular. The distance between the bonethread-free surfaces, 21, 22, 23, and the shaft axis is smaller than thecore radius of bone thread 5 such that surfaces 21, 22, 23 interruptbone thread 5. In all other regards, the design and the use of the boneanchoring element, 1, are identical to the first embodiment.

In order to insert the bone anchoring element 1, according to the fourthembodiment, first a corresponding triangular hole 28 is generated in thebone, as is best seen in FIG. 10 where it is illustrated by the crosshatched area. This can be accomplished for instance by drilling a holefollowed by shaping with a rasp. Alternatively, triangular hole 28 canalso be generated with a transverse cutter or profile-forming cutter.The cross-section of triangular hole 28 corresponds approximately to thecross-section of shaft 3 or is slightly smaller than the cross-section.

Subsequently, shaft 3 of bone anchoring element 1 is inserted intotriangular hole 28 by pressing or soft-hammering such that thetriangular sides of the cross-section through shaft 3 correspondapproximately to the triangular sides of triangular hole 28. In thisarrangement, the bone anchoring element 1 is seated in triangular hole28 firmly to such an extent that it can still be removed by pulling.

Subsequently, bone anchoring element 1 is rotated by a rotating toolengaging on or in the head by a pre-determined angle, preferably 60°(depicted by the dashed line in FIG. 10). As a result, the teeth of bonethread 5 engage the bone and bone anchoring element 1 attains a firmseat.

By rotating further to an angle of 120° or rotating back to the initialposition, bone anchoring element 1 can be loosened, if needed, andremoved from the bone.

As discussed above, bone anchoring element 1 of the fourth embodimentcan be removed from the bone after an extended period of time, when itis firmly surrounded by bone, by unscrewing it with a rotating tool muchlike a conventional bone screw.

Similar to surfaces 11, 12 of the second embodiment, surfaces 21, 22, 23of the fourth embodiment also can be provided to be concave in shapeand/or twisted with respect to each other like surfaces 16, 17 of thethird embodiment.

The bone anchoring element of the invention, such as described in thefirst four embodiments, is ideally suited for the insertion into thepedicle of a vertebral bone, but also is suited for all other bones inwhich a mechanical device is to be anchored. Two types of a connectionbetween a bone anchoring element and a rod are described in thefollowing as examples for a connection of the bone anchoring element toan external device.

FIG. 11 is a schematic representation of a monoaxial connection betweena bone anchoring element 1 and a rod 30.

As is well known to those skilled in the art, the head 2 of boneanchoring element 1 is provided as the receiving member 31 for rod 30and comprises a U-shaped recess 32 by which two free legs 33, 34 areformed. The width of U-shaped recess 32 corresponds approximately to thediameter of rod 30. An internal thread 35 is provided on the inside offree legs 33, 34.

After the insertion of bone anchoring element 1 into the bone, rod 30 isinserted into U-shaped recess 32. Subsequently, a screw 36 is screwedinto internal thread 35 far enough for rod 30 to be fixed between screw36 and the base of U-shaped recess 32.

Any other type of monoaxial connection between shaft 3 and the receivingmember known to those skilled in the art is also useful in the practiceof the present invention. Moreover, the design of the receiving memberfor the rod is not limited to the embodiment shown. For instance, in onemodification, the receiving member does not comprise an internal thread.In this case, the receiving member is connected to a threaded rod andfixed by nuts engaging on the sides. Again, any receiving member designknown to those skilled in the art is useful in the practice of thepresent invention.

FIG. 12 is a schematic representation of a polyaxial connection betweena bone anchoring element 1 and a rod 40.

To provide for the connection, an essentially cylinder-shaped receivingmember 41 with a longitudinal bore 42 is provided. As is well known tothose skilled in the art, the diameter of longitudinal bore 42 isslightly larger than that of the spherical segment-shaped head 2 of boneanchoring element 1. Longitudinal bore 42 extends from a first end ofreceiving member 41 to a second end opposite to the first end. Aspherical segment-shaped section 43, with a diameter of the shape thatis approximately identical to that of spherical segment-shaped head 2 ofbone anchoring element 1, is provided between the second end ofreceiving member 41 and longitudinal bore 42.

In addition, receiving member 41 comprises a U-shaped recess (notshown), whose width corresponds approximately to the diameter of rod 40.This U-shaped recess forms two legs 44, 45 in receiving member 41. Theinside of the legs 44, 45 is provided with an internal thread 46 and theoutside with an external thread 47.

In addition, a sleeve-shaped pressure element 50 is provided. At one endof sleeve-shaped pressure element 50 a spherical segment-shaped recess51 is provided which has approximately the same diameter of the sphereas spherical segment-shaped head 2 of bone anchoring element 1, and atthe other end a cylinder segment-shaped recess 52 is provided which hasapproximately the same diameter as rod 40.

Moreover, for the purpose of fixing the rod and the head, an internalscrew 56, which can be screwed into internal thread 46, and a nut 57,which can be screwed onto external thread 47, are provided.

In operation, shaft 3 of bone anchoring element 1 is inserted intoreceiving member 41 proceeding from the first end of receiving member 41until head 2 of bone anchoring element 1 is held within sphericalsegment-shaped section 43. Subsequently, bone anchoring element 1 isintroduced into the bone. For this purpose, bone anchoring element 1 ispressed or hammered into a hole in the bone that was previouslygenerated, as described above.

Subsequently, the bone anchoring element 1 is rotated by a rotating toolan appropriate amount to fix it in the bone and proceeding from thefirst end of receiving member 41 sleeve-shaped pressure element 50 isslid into longitudinal bore 42 such that spherical segment-shaped recess51 comes to rest on head 2 of bone anchoring element 1. Then rod 30 isinserted into the U-shaped recess such that it comes to rest incylinder-segment shaped recess 52 of pressure element 50.

Alternatively, receiving member and sleeve-shaped pressure element 50are preassembled and anchoring element 1 is rotated by a rotating tool,which is guided through a coaxial bore in sleeve-shaped pressure element50.

Subsequently, screw 56 is screwed into internal thread 46 such that head2 of bone anchoring element 1 is fixed by pressure element 50 and,simultaneously, rod 40 is fixed by screw 56. Finally, nut 57 is screwedonto external thread 47 to secure the fixation.

The polyaxial connection between the head and the receiving memberdescribed above is presented only as an example. Again, any receivingmember and polyaxial connection design known to those skilled in the artis useful in the practice of the present invention. The section 43 ofthe receiving member can be of a shape other than a spherical segment,for example otherwise rounded or conical.

The spherical segment-shaped head illustrated in the first throughfourth embodiment is particularly well-suited for the polyaxialconnection between the bone anchoring element and the external devicedescribed above. However, the bone anchoring element can be providedwith a head of any other shape that allows the external device to beanchored to be connected and/or fixed in a monoaxial or polyaxialfashion. This includes, for example, the provision of a simplecountersunk or fillister head for fixing a plate to a bone.

Alternatively, the shaft of the bone anchoring element can be withouttip. Moreover, it can be slightly conical in shape to ease its insertioninto the bone.

The thread can extend only over a predetermined longitudinal section ofthe shaft rather than over the entire length of the shaft.

The number of bone thread-free surfaces is not limited to two or three.Alternatively, only one surface, or any number of surfaces greater thanthree, can be provided. The number is limited only by the size ofcircumference of the shaft.

The distance of the surfaces from the shaft axis does not necessarilyhave to be smaller than the core radius of bone thread 5. It can also beselected to be identical to or larger than the core radius. In any case,though, is has to be smaller than the distance between the crest of thebone thread and the shaft axis.

The surfaces corresponding to concave surfaces 11, 12 do not have to becurved across their entire width. It is sufficient if merely a part ofthe surface is curved. Alternatively, there can be a v-shaped groove inthe surface or any other geometric configuration that provides foradditional bone growth around the shaft.

In order to improve the in-growth of bone, the surfaces can be roughenedand/or coated, e.g. with hydroxyapatite.

Moreover, the bone anchoring element can be provided with a longitudinalbore which extends through its head and is open or closed at the tip,and bores which branch off from the longitudinal bore in a lateraldirection and open on the lateral walls of the shaft, said boresallowing for a medication or bone cement to be injected into theattachment bore.

In a further embodiment, the bone anchoring element can be made from ashape memory alloy, e.g., nitinol™. In this case, the bone anchoringelement may be provided, e.g., such that, at room temperature, itassumes the shape corresponding to FIG. 1, in which the bone thread-freesurfaces are planar and parallel to each other, and, at bodytemperature, assumes the shape corresponding to FIG. 7, in which thesurfaces are twisted with respect to each other.

During insertion at room temperature, the parallel surfaces allow foreasy insertion of the bone anchoring element. Once inside the body, theshape of the bone anchoring element changes such that the surfacesbecome twisted and additional stability in the bone can be achieved.

The bone anchoring element made from the shape memory alloy also can beformed such that the teeth of the bone thread are smaller at roomtemperature than at body temperature. As a result, the teeth of the bonethread grow once they are inside the body such that additional stabilityin the bone can be achieved.

In a second embodiment shown in FIG. 13 the bone anchoring element 100comprises a shaft, but lacks a head. The shaft has a section 104 whichis formed like the shaft 3 of the bone anchoring elements of thepreviously described embodiments with bone thread sections 105 and bonethread-free sections 106. Adjacent to section 103 the bone anchoringelement comprises a cylindrical section without bone thread. Such boneanchoring element is used e.g. in place of a usual Schanz screw or pinscrew.

FIG. 14 shows an example of application of the bone anchoring element100 in a Fixateur externe to be used for example for stabilizingfractures of long bones 150. The bone ancoring element is anchored withsection 103 in the long bone and connected to a receiving element 108for receiving a rod 109.

In a third embodiment shown in FIG. 15 the bone anchoring element 110 isformed as a marrow nail to be anchored in a long bone 111 such as femuror humerus. The bone anchoring element comprises a shaft having asection 114 which is is formed like the shaft 3 of the bone anchoringelements of the first and second embodiment with bone thread sectionsand bone thread-free sections. The shaft further has a bone thread freesection 115 and a section 116 through which a bone screw 117 is screwedfor fixation. In the example shown there is no external device to beconnected.

The invention has been described in detail, including the preferredembodiments thereof. However, it will be appreciated that those skilledin the art may make modifications and improvements within the spirit andscope of the invention.

1. A bone anchoring element for anchoring an external device in a bone,the bone anchoring element comprising: a head section that can beconnected to the external device; and a shaft that can be anchored inthe bone and is connected to the head section, having a longitudinalaxis; the shaft comprising: a bone thread on a predetermined section ofthe shaft, the bone thread having a crest, and a first end and a secondend spaced apart longitudinally along the shaft, and at least one bonethread-free surface that extends along the direction of the shaft axisfrom the first end to the second end of the bone thread.
 2. The boneanchoring element according to claim 1, wherein a first distance betweenthe bone thread-free surface and the shaft axis is smaller than a seconddistance between the crest of the bone thread and the shaft axis.
 3. Thebone anchoring element according to claim 1, further comprising aplurality of bone thread-free surfaces.
 4. The bone anchoring elementaccording to claim 3, wherein the shaft further comprises acircumference and wherein the plurality of bone thread-free surfaces arepositioned approximately at equal distances from each other around theshaft circumference.
 5. The bone anchoring element according to claim 1,further comprising two or three bone thread-free surfaces.
 6. The boneanchoring element according to claim 1, wherein at least one bonethread-free surface has a geometry such that a cross section of theshaft perpendicular to the longitudinal axis of the shaft shows thesurface as a straight line.
 7. The bone anchoring element according toclaim 1, wherein at least one bone thread-free surface has a concaveshape.
 8. The bone anchoring element according to claim 1, wherein thebone thread-free surface extends parallel to the shaft axis.
 9. The boneanchoring element according to claim 1, wherein the bone thread-freesurface extends from the first end to the second end of the bone threadin a helical form.
 10. The bone anchoring element according to claim 1,wherein the shaft has an end opposite to the head section and thepredetermined section extends to the end of the shaft.
 11. The boneanchoring element according to claim 1, the shaft further comprising anend opposite to the head section, the end comprising a tip.
 12. The boneanchoring element according to claim 1, wherein at least one bonethread-free surface is roughened or coated in order to improve thein-growth of bone.
 13. The bone anchoring element according to claim 1,wherein the shaft comprises a shape memory alloy.
 14. The bone anchoringelement according to claim 1, wherein the head section comprises areceiving member for connecting to a rod.
 15. The bone anchoring elementaccording to claim 1, wherein the head section is fixedly connected tothe shaft.
 16. The bone anchoring element according to claim 1, whereinthe head section is connected to the shaft in an articulatedarrangement. 17-22. (canceled)
 23. A bone anchoring element comprising ashaft to be anchored in the bone, the shaft having a longitudinal axis;wherein the shaft comprises a bone thread on a predetermined section ofthe shaft, the bone thread having a crest and a first end and a secondend spaced apart longitudinally along the shaft, and at least one bonethread free surface that extends along the direction of the shaft axisfrom the first end to the second end of the bone thread.
 24. The boneanchoring element of claim 23, wherein the bone anchoring element isformed as a pin.
 25. The bone anchoring element of claim 23, wherein thebone anchoring element is formed as a marrow nail.